1. Field of the Invention
The present invention relates to an optical pickup mounted on a DVD recorder and the like, and in particular, to an optical pickup including a liquid crystal element having an electrode pattern for correcting aberration.
2. Description of the Related Art
In an optical pickup for performing recordation and reproduction of information on the optical disc such as a CD (Compact Disc), a DVD (Digital Versatile Disc), and a BD (Blu-ray Disc; registered trademark), the specification of objective lens and light source differs depending on the type of optical disc. For instance, the numerical aperture (NA) of the objective lens is 0.50 for a CD, 0.65 for a DVD, and 0.85 for a BD, and the wavelength of the laser light is 780 nm for a CD, 650 nm for a DVD, and 405 nm for a BD.
As mentioned above, the numerical aperture of the objective lens and the wavelength of the laser light differ depending on the type of optical disc. If different optical pickup is used for each disc, the number of components increases thereby leading to enlargement of device and increase in cost. Therefore, an optical pickup compatible to a plurality of wavelengths that can correspond to various optical discs with one optical pickup is being developed. In order to reduce the number of components, enhance the assembly workability, and achieve miniaturization, the optical pickup mounted with only one objective lens is also being put to practical use.
However, when performing recordation and reproduction on a plurality of types of optical discs with one objective lens, the thickness of the protective layer which protects the recording layer of the disc differs depending on the type of optical disc, which becomes a cause of occurrence of spherical aberration in the optical system. Such spherical aberration degrades the optical spot formed on a recording layer of the optical disc, and lowers the recordation and reproduction performance. Furthermore, the distance from the objective lens to the protective layer, that is, the working distance in a case where the light beam is collected on the recording layer by the objective lens becomes particularly small for a CD due to the difference in thickness of the protective layer, thereby rising a problem of collision of the objective lens with the optical disc.
FIGS. 6A to 6C are views describing the problem of spherical aberration and working distance. FIG. 6A shows a case where the optical disc is a BD, where 101 is the recording layer and 102 is the protective layer. FIG. 6B shows a case where the optical disc is a DVD, where 201 is the recording layer and 202 is the protective layer. FIG. 6C shows a case where the optical disc is a CD, where 301 is the recording layer and 302 is the protective layer. A is the objective lens, L1 to L3 are light beams (laser lights) of each wavelength, and WD1 to WD3 are working distances. Here, assuming that the objective lens A is suitably designed for a BD, spherical aberration does not occur for a ED, but spherical aberration occurs for a DVD and a CD since the protective layers 202, 302 are thicker than the protective layer 102. Even for a BD, correction of spherical aberration is required if a BD has a plurality of recording layers. Furthermore, the working distance WD3 becomes very small for a CD having the thickest protective layer 302, and the objective lens A might collide with the disc surface.
As shown in FIG. 7, when recording and reproducing a CD, it is known that a liquid crystal element B including an electrode configuring a diffraction pattern is electrically controlled, and the light beam L3 is diverged by an angle α so as to enter the objective lens A as divergent light L3′, thereby correcting the spherical aberration (see e.g., Japanese Unexamined Patent Publication No. 2006-252655). In this case, since the divergent light L3′ enters the objective lens A, a large working distance WD3′ can be ensured compared to that in FIG. 6C (WD3′>WD3), and the objective lens A is avoided from colliding with the disc surface. However, the spherical aberration of a BD having a plurality of recording layers cannot be corrected with only the means of FIG. 7.
It is known that spherical aberration can be corrected by electrically controlling the liquid crystal element including an electrode configuring a phase shift pattern and providing a phase difference to the light beam entered to the objective lens (see e.g., Japanese Unexamined Patent Publication No. 2006-12344 and Japanese Unexamined Patent Publication No. 2005-202323). Through the use of such a method, the spherical aberration can be corrected even for a BD having a plurality of recording layers, but two liquid crystal elements, one for generating divergent light and the other for phase shift, are required to ensure the working distance while correcting the spherical aberration of a CD and to correct the spherical aberration of each recording layer of a BD, which leads to increase in number of components and increase in cost.
The applicant thus proposed an optical pickup capable of correcting the spherical aberration and ensuring the working distance in a CD, and also capable of correcting the spherical aberration in each recording layer of a BD with one liquid crystal element (Japanese Patent Application No. 2006-227900). FIGS. 8 and 9 show the liquid crystal element according to the above previous application. The liquid crystal element 60 includes a concentric electrode pattern 64, where an electrode 66 of a diffraction pattern for generating the divergent light is arranged in a first region X on the inner side, and an electrode 67 of phase shift pattern is arranged in a second region Y on the outer side. A pair of substrates 61 and 62, a liquid crystal 63, and a common electrode 65 are arranged. Through the use of such liquid crystal element, the spherical aberration can be corrected and the working distance can be increased for a CD since divergent light is generated similarly to the conventional art by applying voltage to the electrode 66 of diffraction pattern. In a case of a BD, the spherical aberration of each recording layer of a BD can be corrected by turning OFF the voltage of the electrode 66 of diffraction pattern, and appropriately controlling the voltage of the electrode 67 of phase shift pattern.
FIGS. 10A and 10B are diagrams describing the correction of spherical aberration by the phase shift pattern of the second region Y. The heavy solid line of FIG. 10A shows the spherical aberration that occurs in the light beam when reproducing a BD. As shown in the figure, the spherical aberration becomes large at the outer peripheral side distant from the optical axis. Therefore, the degradation of reproduction quality caused by the spherical aberration can be suppressed by correcting the large spherical aberration that occurs mainly on the outer peripheral side. For this purpose, the number and area of the concentric region of the phase shift electrodes 67 in the second region Y should be set to values which can correct the spherical aberration that becomes larger towards the outer periphery. The thin solid line of FIG. 10A shows a correction pattern in a case where correcting the spherical aberration by adjusting the application voltage to be applied to each region, using a plurality of concentric regions which number and area of the region are determined as described above.
The heavy solid line of FIG. 10B shows the spherical aberration after correction by subtracting the correction pattern from the spherical aberration of FIG. 10A. Apparently, the spherical aberration can be reduced by performing the correction of changing the phase distribution in the second region Y. Hence, even in a case of a BD including a plurality of recording layers, the correction of the spherical aberration can be easily performed by voltage control of the phase shift electrodes 67.
However, in the case of the liquid crystal element 60, phase shift at the region X is impossible since the electrode 66 of the diffraction pattern and the electrode 67 of the phase shift pattern are concentrically arranged on the same substrate 61. Thus, the correction residual shown in FIG. 10B becomes large, and there is a limit to obtaining a satisfactory reproduction signal.
Japanese Unexamined Patent Publication No. 2006-286028 describes forming a concentric diffraction pattern on a pair of opposing substrates in the liquid crystal element for correcting spherical aberration, but the phase shift pattern is not referenced. Furthermore, Japanese Unexamined Patent Publication No. 2004-178773 describes forming an electrode pattern for correcting the spherical aberration of the BD on one substrate and forming an electrode pattern for correcting the spherical aberration of the DVD etc. on the other substrate, but there is a limit to further reducing the correction residual by simply arranging different electrode patterns separately on the substrate.